1. Technical Field
The present invention generally relates to devices and methods for using DC-DC converters and for converting a continuous signal of a given value into another signal of another value, for example for envelope tracking or polar modulation for radio frequency power amplifiers. The present invention finds applications in particular in electronic portable devices such as for mobile phones, digital walk-men, portable computers, PDA, etc.
2. Related Art
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Multiple supply voltages have become increasingly common due to the coexistence of low-power digital circuits, high-speed digital circuits, and analog/RF circuits on a single die. DC-DC converters can be realized as linear, switched-capacitor, and inductive converters.
There are known inductive Step-Down converters as depicted in FIG. 1. Such converters may be used to provide the best efficiency for a wide range of conversion ratios but have higher output ripple than linear converters. As they produce the output signal as a pulse-width modulated (PWM) signal by switching between a supplied voltage of the battery and a ground voltage value, a high level of ripples is generally obtained, even after filtering. The switching can be attained by opening and closing alternatively (and in inverse position) two switches S1 and S2 situated on the connection to the battery or to the ground.
Low ripple behaviour is however a stringent parameter, especially for DC-DC converters dedicated to supply power for radio-frequency (RF) applications, because the ripples may create interferences, with RF signals. They thus decrease the signal-to-noise ratio of the RF signal carried by the circuit.
For a given output capacitor value (Cout) and a given inductor (LSD) in a DC-DC converter, ripple reduction schemes traditionally rely either on the use of high-value, off-chip inductors or on the increase of the switching frequency as shown in equation 1.
                              Output          ⁢                                          ⁢          Ripple                =                                            D              ·                              (                                  1                  -                  D                                )                                                    8              ·                              L                SD                            ·                              C                OUT                            ·                              f                SW                2                                              ⁢                      V            BAT                                              Eq        .                                  ⁢        1            where Vbat is the supply voltage from the battery and D is a working duty-cycle of the PWM signal linked to the output voltage.
This approach is currently used for classic DC-DC converters, where the output voltage is not varying (the output voltage is a DC value converted from Vbat), but this is not appropriate for RF applications wherein the output voltage is amplitude modulated. This is the case for both signal amplification by envelop tracking and by polar modulation.
These applications rely on quickly changing voltage regulators. In RF power amplifiers, efficiency varies with the RF signal amplitude. Usually the yield is maximal at full power and it drops rapidly when the amplitude of the RF signal decreases, as a larger part of the supply voltage is not used and is thus dissipated in transistors. This drawback may be partly compensated by tracking the signal amplitude with an efficient power supply modulator that has a quick response.
In such systems, as shown in FIG. 2, an efficient, WIDE-BANDWIDTH envelope-tracking power supply 20 modulates the supply voltage for the RF power amplification (RFPA).
In order to track the envelope of high-bandwidth modulated signal, a DC-DC converter commonly uses a low pass filter, typically of LC type, positioned near an output terminal, with a cut-off frequency higher than the modulation bandwidth. This relies on the use of a small inductor (or small capacitor) as shown by Eq.2.
                              LC          ⁢                                          ⁢          filter          ⁢                                          ⁢          bandwidth                =                              1                          2              ⁢              π              ⁢                                                                    L                    SD                                    ⁢                                      C                    OUT                                                                                >                      Modulation            ⁢                                                  ⁢            Bandwidth                                              (                  Eq          .                                          ⁢          2                )            
As a result, a compromise has generally to be found between the amplitude of the ripples outputted, the bandwidth of the converter and the switching frequency fsw of the PWM signal.
In a standard Step-Down DC-DC converter as illustrated in relation to FIG. 1 two equations have generally to be optimized in order to reduce the outputted ripples, achieve the necessary bandwidth to perform Envelope Tracking or Polar Modulation applications:
                              Output          ⁢                                          ⁢          Ripple                =                                                            D                ·                                  (                                      1                    -                    D                                    )                                                            8                ·                                  L                  SD                                ·                                  C                  OUT                                ·                                  f                  SW                  2                                                      ⁢                          V              BAT                                ≤                      Maximum            ⁢                                                  ⁢            Ripple            ⁢                                                  ⁢            Allowed                                              (                  Eq          .                                          ⁢          3                )                                          LC          ⁢                                          ⁢          filter          ⁢                                          ⁢          bandwidth                =                              1                          2              ⁢              π              ⁢                                                                    L                    SD                                    ⁢                                      C                    OUT                                                                                ≥                      Modulation            ⁢                                                  ⁢            Bandwidth                                              (                  Eq          .                                          ⁢          4                )            
As a result the easiest solution is to increase the switching frequency fsw to reduce the outputted ripples and dimension the output filter so as to cope with bandwidth specifications.
But, nonetheless, this solution presents some issues. For example, the Standard Step-Down DC-DC used at a high switching frequency, has an efficiency frequently lower than 70% due to the high switching frequency. Furthermore, the high frequency may create interferences with RF signals that may be carried in the circuit, or that may be amplified using the output voltage. Eventually, the standard step-down converter cannot output a voltage superior to the supply voltage.
Alternative solutions may use different architecture instead of a classic one.
A standard Step-Down DC-DC, using an AB class amplifier to compensate has also an efficiency that is frequently under 70% due to the use of the amplifier. Furthermore, it proves difficult to control the current sharing between the converter and the amplifier.
An Interleaved Step-Down converter also has some few limitations. Its efficiency is reduced due to the use of two power stages each having at least one power coil. There are difficulties to maintain a good synchronization needed between 4 phases. Eventually, it cannot output a voltage superior to the supply voltage.